A multi-carrier communication system, such as a Discrete Multi-Tone (DMT) system in the various types of Digital Subscriber Line (DSL), for example, asymmetric digital subscriber line (ADSL) and very high-speed digital subscriber line (VDSL) systems, carries an information bit stream from a transmitter to a receiver. The information bit stream is typically converted into a sequence of data symbols having a number of tones. Each tone may be a group of one or more frequencies defined by a center frequency and a set bandwidth. The tones are also commonly referred to as sub-carriers or sub-channels. Each tone acts as a separate communication channel to carry information between a local transmitter-receiver (transceiver) device and a remote transceiver device.
FIG. 1 is a block diagram illustrating a conventional DMT receiver. A channel equalizer is used to control the spread of the data symbols after going through the channel. A cyclic prefix (CP) may be employed in such systems to simplify channel equalization to minimize a source of cross channel interference. Generally, if the length of the channel impulse response is equal to or less than the cyclic prefix length plus one sample, then channel equalization is trivial and perfect equalization can be achieved. The channel can be inverted in the frequency domain after a discrete Fourier transform (DFT) by a single complex multiply for each sub-channel. This is usually referred to as frequency-domain equalization (FEQ).
On transmission lines in DMT communication systems, such as ADSL or VDSL, the data signal is generally transmitted differentially. Interference such as radio-frequency interference (RFI), crosstalk and impulse noise electromagnetically couples into both the common mode and the differential mode of such transmission lines. In the case of a binder containing multiple transmission lines, such interference may couple into some or all of the transmission line in the binder and such noise may be correlated between lines.
Conventional techniques for reducing differential noise, thereby improving data rates over the DSL, include use of common-mode information. In a traditional DSL system, the common-mode voltage is measured, an estimate of the differential-mode interference is constructed and the interference estimate is subtracted from the desired signal.
Traditional cancellation may occur in the time-domain or the frequency domain. For example, frequency bands containing RFI are band-pass filtered and then subtracted from the differential-mode signal in the time domain. In the frequency domain, a small set of frequency bins are used to compute and remove an estimate of RFI on a larger number of data carrying frequency bins. Other conventional systems cancel crosstalk in both the time domain and the frequency domain by solving a specific set of equations.
However, there are significant drawbacks associated with filtering and subtracting an interference estimate in the time-domain. For example, training and updating the noise estimation unit is difficult, especially in the presence of a data signal. Furthermore, time-domain subtraction tends to result in noise enhancement. A reduction in the power spectral density (PSD) of the interference may be achieved over parts of the frequency band where the interference is strongest, but interference PSD enhancement may occur in other frequency regions, resulting in sub-optimal system performance.
Known frequency-domain techniques also have significant limitations. Common-mode interference may not be limited to crosstalk or RFI alone, but may be a combination of the two. There may also be wideband noise from sources other than radio transmitters (RFI) or other communications systems (crosstalk) that is correlated between the common and differential modes. Conventional solutions are suited to target only crosstalk or RFI; not both. Also, the interference sources and their associated coupling transfer functions will, in general, change over time. Known cancellers do not have the ability to adapt to these changing conditions in the presence of the data signal. Furthermore, in a practical implementation, there are complications and difficulties associated with the dynamic range of both the differential-mode and common-mode signals. In implementations in which the multi-carrier communications system is an ADSL or VDSL system, there may be further complications involving interaction of the canceller with On-Line Reconfiguration (OLR), Seamless Rate Adaptation (SRA), and bitswap as defined in the various ADSL and VDSL standards. During such events the transmitted power and/or the constellation size changes for one or more sub-carriers.